To provide a cylinder device, in which degassing can be efficiently performed without increasing the size of the device.

On an end part peripheral wall in the axial direction of a cylinder 3, a degassing hole 24 is formed at a position on an upper side in a mounted state, a C-shaped ring 52 is disposed along the outer periphery of the cylinder 3 to cover the degassing hole 24, and a space 53 is formed between the C-shaped ring 52 and the outer periphery of the cylinder 3 to communicate with the degassing hole 24. As the C-shaped ring 52 has a discharge region in which the space 53 is communicatable with operating fluid in a reservoir chamber 6 in a mounted state, degassing can be efficiently performed without increasing the size of the device.